There is a conventionally known a humidity indicator of the above-described type, having a structure including a humidity-determining face which is provided on a surface of a humidity-determining plate, for example, comprising cobalt chloride held in a base paper sheet, so that the cobalt chloride is exposed to the humidity-determining face, whereby the determination of humidity around the base paper sheet can be visually conducted by the discoloration (from blue to pink) of the cobalt chloride on the humidity-determining face.
Such a humidity indicator is used in a state in which it has been sealedly contained in a transparent air-tight packaging bag along with a variety of industrial products adversely affected by moisture (e.g., electronic parts such as circuit boards which may be cracked by the absorption of moisture by an epoxy resin) and a drying agent in the transportation of the industrial products. In the transportation of the products in such used state, whether or not the humidity in the packaging bag exceeds a defined limit can be visually determined by the color of the humidity-determining face of the humidity indicator. Therefore, whether the inside of the packaging bag with the drying agent contained therein is kept in an appropriate humidity state (a dried state) can be checked easily and hence, such humidity indicator is conventionally widely used in an electronic industry and the like.
In the conventional humidity indicator, both of the surface and back of the humidity-determining plate (hence, even the humidity-determining face to which the cobalt chloride is exposed) are placed in states exposed to the outside, and an operator picks the humidity-determining plate directly with his or her hand and withdraws and places it out of and into a dry pack.